ucb1100 NXP Semiconductors, ucb1100 Datasheet - Page 16

• Manufacturer Part Number: ucb1100
• Description: Advanced Modem/audio Analog Front-end
• Manufacturer: NXP Semiconductors

Philips Semiconductors
6.3 Touch Screen Measurement Modes
The UCB1100 contains an on chip interface for a 4 wire resistive
touch screen. This interface supports three modes of touch screen
measurements, position, pressure and plate resistance.
6.3.1 Position Measurement
Two position measurements are needed to determine the location of
the pressed spot. First an X measurement, secondly a Y
measurement. The X plate is biased during the X position
measurement the X plate and the voltage on one or both Y terminals
(tspy, tsmy) measured. The circuit can been represented by a
potentiometer, with the tspy and/or tsmy electrode being the ‘wiper’.
The measured voltage on the tspy/tsmy terminal is proportional to
the X position of the pressed spot of the touch screen.
In the Y position mode the X plate and Y plate terminals are
interchanged, thus the Y plate is biased on the voltage on the tspx
and/or tsmx terminal is measured.
6.3.2 Pressure Measurement
The pressure used to press the touch screen can be determined. In
fact the contact resistance between the X and Y plate is measured,
which is a good indication of the size of the pressed spot and the
applied pressure. A soft stylus, e.g. a finger, leads to a rather large
contact area between the two plates when a large pressure is
applied. A hard stylus, e.g. a pen, leads to less variation in
measured contact resistance since the contact area is rather small.
One plate is biased at one or both terminals during this pressure
measurement, whereas the other plate is grounded, again on one or
both terminals. The current flowing through the touch screen is a
direct indication for the resistance between both plates. A
compensation for the series resistance, formed by the touch screen
plates itself will improve the accuracy of this measurement.
1998 May 08
Figure 15. Touch Screen Setup for Pressure Measurement
Advanced modem/audio analog front-end
Figure 14. Touch Screen Setup for Position Measurement
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Î Î Î Î Î Î Î Î Î Î Î Î
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Î Î Î Î Î Î Î Î Î Î Î Î
Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï
Î Î Î Î Î Î Î Î Î Î Î Î
tsmy
tsmy
Vposition
Vtscbias ipresure
tsmx
Vtscbias
tsmx
Xplate
Xplate
tspx
tspx
SN00139
SN00140
tspy
tspy
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6.3.3 Plate Resistance Measurement
The plate resistance of a touch screen varies typically a lot due to
processing spreads. Knowing the actual plate resistance makes it
possible to compensate for the plate resistance effects in the
pressure resistance measurements. Secondly the plate resistance
decreases when two or more spots on the touch screen are
pressed. In that case a part of one plate, e.g. the X plate is shorted
by the other plate, which decreases the actual plate resistance.
The plate resistance measurement is executed in the same way as
the pressure resistance measurement. In this case only one of the
two plates is biased and the other plate is kept floating. The current
through the connected plate is again a direct indication of the
connected resistance.
Figure 16. Touch Screen Setup for Plate Resistance
Vtscbias iplate
tsmy
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Î Î Î Î Î Î Î Î Î Î Î Î
Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï Ï
Î Î Î Î Î Î Î Î Î Î Î Î
Measurement
tsmx
Xplate
tspx
Preliminary specification
UCB1100
SN00141
tspy